Semiconductor injection laser

ABSTRACT

A semiconductor injection laser which emits a beam of radiation having improved beam divergence has a body of semiconductor material including a first region of one conductivity type, a second region of the opposite conductivity type and a third region of either conductivity type between the first and second regions and forming a PN junction with one of the first or second regions. The junctions between the third region and each of the first and second regions are heterojunctions which extend in substantially parallel relation to the edges of the body. The third region is of a thickness of between 0.2 and 0.3 microns and has an index of refraction which is no greater than about 0.05 higher than the index of refraction of each of the first and second regions so as to permit some of the light generated in the third region to spread out into each of the first and second regions.

United States Patent [191 Kressel et a1.

[ SEMICONDUCTOR INJECTION LASER [75] Inventors: Henry Kressel,Elizabeth; Frank Zygmunt Hawrylo, Trenton, both of [73] Assignee: RCACorporation, New York, NY. 22 Filed: Aug. 26, 1971 I [21] Appl. No.:175,197

[52] US. Cl. 331/945 H, 148/175, 317/235 R OTHER PUBLICATIONS Butler etal. "High Order Transverse Cavity Modes in l-leterojunction Lasers,"Applied Physics Letters, Vol. 17, pp. 403-406, November 1970. Dyment:Hermite-Gaussien Mode Patterns in GaAs Junction Lasers, Applied PhysicsLetters, Vol. 10, pp.

[451 Jul 17,1973

tinuously at Room Temperature," Applied Physics Letters, Vol. 17, pp.109-111, Aug. 1, 1970.

Primary Examiner-Edward S. Bauer Attorney-Glenn l-l. Bruestle [5 7]ABSTRACT A semiconductor injection laser which emits a beam of radiationhaving improved beam divergence has a body of semiconductor materialincluding a first region of one conductivity type, a second region ofthe opposite conductivity type and a third region of either conductivitytype between the first and second regions and forming a PN junction withone of the first or second regions. The junctions between the thirdregion and each of the first and second regions are heterojunctionswhich extend in substantially parallel relation to the edges of thebody. The third region is of a thickness of between 0.2 and 0.3 micronsand has an index of refraction which is no greater than about 0.05higher than the index of refraction of each of the first and secondregions so as to permit some of the light generated in the third regionto spread out into each of the first and second regions.

8 Claims, 3 Drawing Figures 1. SEMICONDUCTOR INJECTION LASER BACKGROUNDOF THE INVENTION The invention herein disclosed was made in the courseof or under a contract or subcontract thereunder with the Department ofthe Army.

The present invention relates to a semiconductor injection laser, and,more particularly to a semiconductor injection laser having a reducedbeam divergence.

Semiconductor injection lasers, in general, are bodies of a singlecrystalline semiconductor material which, when biased, emit light,either visible or infrared, through the recombination of pairs ofoppositely charged carriers. Such devices generally include regions ofopposite conductivity type forming a PN junction therebetween. When thejunction is properly biased, charge carriers of one type are injectedfrom one of the regions into the other where the predominant chargecarriers are of the opposite type so as to achieve the light generatingrecombination.

To provide a semiconductor injection laser which is capable of efficientemission of stimulated radiation at room temperature, various structureshave been devised which include an optically confining cavity betweenregions of opposite conductivity type in which the generation ofradiation by the recombination of the charge carriers occurs. The cavityis generally a narrow region extending across the semiconductor bodybetween the ends and side edges of the body. Optical confinement isusually achieved by making the regions of the body on each side of thecavity of a material having an index of refraction lower than that ofthe material of the cavity. The side edges and one end edge of the bodyis made reflective and the other end edge is made partially transmittingso as to form a Fabry-Perot cavity. Thus, the radiation generated in theoptically confining cavity is emitted from the partially transmittingend edge of the body as a beam of coherent radiation. Some structures ofsemiconductor injection lasers having optically confining cavities aredescribed in the articles Close-Confinement Gallium Arsenide PN JunctionLasers with Reduced Optical Loss at Room Temperature by H. Kressel etal., RCA REVIEW, Volume 30 No. 1, pages 106-1 13, March, 1969,High-Order Transverse Cavity Modes in Heterojunction Diode Lasers by J.Butler et al., APPLIED PHYSICS LET- TERS, Vol. 17, No.9, Nov. 1, 1970,pgs. 403-406, and An Efficient Large Optical Cavity Injection Laser by H.F Lockwood et al., APPLIED PHYSICS. LETTERS, Vol. 17, No. 12 Dec. 1,1970, pgs. 499-502.

A problem with such optically confining cavity semiconductor injectionlasers is the divergence of the beam of radiation emitted by the lasers.By divergence of the beam is meant that the beam spreads out as it movesaway from the emitting end edge of the laser. Thus, the cross-sectionalarea of the beam in a plane perpendicular to the PN junction of thelaser increases along the beam in the direction away from the laser.

in the use of semiconductor lasers the emitted beam of light is directedat a target and it is desirable that the beam contact the target as aspot of controlled area. Therefore, it is desirable that the emittedbeam be columnar, i.e., of uniform cross-sectional area along itslength, or at least have a minimum of divergence so as to simplify thelensing system which may be needed in the optical system between thesemiconductor laser and the target to achieve the desired spot of lightat the target. It has been found that merely changing the thickness ofthe optically confining cavity does not reduce the beam divergence inthe direction perpendicular to the junction beyond a minimum amount. Forexample, a semiconductor injection laser having an optically confiningcavity of about 2 microns in thickness provides an emitted beam ofradiation having a divergence of about 20 in the direction perpendicularto the junction. If the cavity is made thinner, the divergence of theemitted beam increases, and if the cavity is made thicker, there iscreated a second propagating mode of the generated light which causesthe emission of two or more separate divergent beams of radiation ratherthan a single beam.

SUMMARY OF THE INVENTION A semiconductor injection laser including abody of single crystalline semiconductor material having a first regionof one conductivity type, a second region of a conductivity typeopposite to that of the first region and a third region of eitherconductivity type between the first and second regions. The junctionsbetween the third region and each of the first and second regions aresubstantially parallel and extend to an edge of the body. The thirdregion is of a thickness of between 0.2 and 0.3 microns and has an indexof refraction at the lasing wavelength which is slightly higher than theindex of refraction of each of the first and second regions.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a cross-sectional view of asemiconductor injection laser in accordance with the present invention.

FIG. 2 is a diagram showing the intensity and position of the lightgenerated in the semiconductor injection laser shown in FIG. 1 at theemitting surface of the laser.

FIG. 3 is a cross-sectional view of an apparatus for making thesemiconductor injection laser.

THE PREFERRED EMBODIMENT STRUCTURE Referring initially to FIG. 1, thesemiconductor injection laser of the present invention is generallydesignated as 10. The semiconductor injection laser 10 comprises a flatsubstrate 12 of an N type single crystalline semiconductor material,such as gallium arsenide having a dopant concentration of about l0 cm.On a surface of the substrate 12 is a first region 14 of singlecrystalline N type semiconductor material. A second region 16 of singlecrystalline P type semiconductor material is over the first region 14,and a thin third region 18 of single crystalline semiconductor materialis between the first region 14 and the second region 16.

The third region 18 is ofa thickness between 0.2 and 0.3 microns. Thejunctions 20 and 22 between the third region 18 and each of the firstand second regions 14 and 16, respectively, are heterojunctions andextend in substantially parallel relation to one another to the edges ofthe semiconductor injection laser 10. The third region 18 is preferablyof P type conductivity although it can also be of N type conductivity.The third region 18 is preferably heavily doped and closely compensatedwith both P type and N type dopants. The total concentration of thedopants in the third region 18 is preferably about 10"cm with thedifference in the dopant concentration between the P type and N typedopants being about l0 cm If the third region 18 is of P typeconductivity, the concentration of the P type dopant is higher than thatof the N type dopant and vice versa.

Most importantly, the third region 18 is of a semiconductor materialwhich has an index of refraction which is only slightly higher than theindex of refraction of the semiconductor material of each of the firstand second regions 14 and 16. The difference in the index of refractionshould be no greater than about 0.05 at the lasing wavelength of thelaser 10. This difference in the index of refraction can be achieved bymaking the third region 18 of a semiconductor material which has a bandgap energy which is slightly lower than the band gap energy of thesemiconductor materials of the first and second regions 14 and 16. Adifference in the band gap energies of no greater than about 0.2 eV willprovide the desired difference in the index of refraction.

There are various semiconductor materials, particularly among the groupIII-V semiconductor compounds and alloys thereof, which have differentband gap energies and which can be used for the regions of the laser 10.For example, the third region 18 can be made of gallium arsenide whichis doped with silicon, an amphoteric dopant, and zinc, as the P typedopant, in the proper amounts to provide a compensated region of thedesired conductivity type specified above. The first and second regions14 and 16 can be made of gallium aluminum arsenide which has a higherband gap energy than gallium arsenide. The band gap energy of galliumaluminum arsenide can be varied by varying the amount of aluminum in thecompound. Thus, by making the amount of aluminum in the compound small,i.e., Ga ,Al,As where x is between 0.1 and 0.15, the desired band gapenergy difference between the third region 18 and each of the first andsecond regions 14 and 16 can be achieved. The material of the firstregion 14 can contain tellurium as the N type dopant and the material ofthe second region can contain zinc as the P type dopant. The dopantconcentration in each of the first and second regions 14 and 16 ispreferably about lO cm' Alternatively, the third region 18, as well asthe first and second regions 14 and 16, can be made of gallium aluminumarsenide, with the amount of aluminum in the material of the thirdregion 18 being slightly less than the amount of aluminum in thematerials of the first and second regions 14 and 16 so as to provide therequired difference in the band gap energies of the regions.

The semiconductor injection laser is generally in the form of arectangular parallelopiped. It is formed into a Fabry-Perot cavity bymaking a pair of opposite side edges and one end edge reflective and theother end edge partially transparent. Terminals are attached to thesecond region 16 and to the substrate 12 to permit the laser 10 to beconnected to a suitable voltage source.

GENERAL OPERATION Considering a semiconductor injection laser 10 inwhich the third region 18 is of P type conductivity, the PN junction ofthe laser 10 is the junction between the third region 18 and the firstregion 14. Upon the application of a forward bias voltage to the PNjunction 20, electrons are injected from the N type first region 14 intothe P type third region 18 and holes from region 16. The injectionelectrons undergo radiative recombination in the third region 18 withthe result that light is generated in the third region 18. Since thethird region 18 has an index of refraction higher than the index ofrefraction of each of the first and second regions 14 and 16, asignificant portion of the generated light is confined within the thirdregion 18. However, since the difference in the index of refraction issmall, some of the light spreads out into each of the first and secondregions 14 and 16. The light in the third region 18 propagates along thethird region and is emitted from the partially transparent end edge ofthe laser 10. Laser action, i.e., the emission of stimulated radiation,is obtained by the application of sufficient voltage to produce acurrent density in excess of the lasing threshold value. As shown inFIG. 2, above the lasing threshold, the intensity of the generated lightis symmetrically distributed with the portion of maximum intensity beingwithin the third region 18 and the intensity of the light diminishinginto each of the first and second regions 14 and 16.

In the semiconductor injection laser 10, by allowing some of the lightgenerated in the third region 18 to spread out into the first and secondregions 14 and 16, the beam of stimulated radiation emitted from thelaser 10 at a given laser threshold value has a divergence less thanthat of the beams emitted by other types of optically confining cavitysemiconductor injection lasers operating at the same threshold value.Reductions in beam divergence by a factor of greater than two have beenachieved by the semiconductor injection laser 10 while maintaining lowthresholds and relatively high efficiencies.

Thus, the semiconductor injection laser 10 provides a more columnar beamof light so as to permit the use of a more simplified optical systembetween the laser and a target in order to provide a small spot of lighton the target. By having the third region 18 of a thickness of between0.2 and 0.3 microns, the lasing threshold of the laser is minimized.

A semiconductor injection laser 10 having an N type third region 18operates in the same manner as described above except that the PNjunction is the junction 22 between the third region 18 and the secondregion 16. Also the generation of light results from the injection ofelectrons and holes from the first and second regions 14 and 16respectively into the third region 18 where the recombination takesplace.

METHOD OF MAKING PREFERRED EMBODIMENT The semiconductor injection laser10 may be made by epitaxially depositing the regions on the substrate 12with the first region 14 being deposited first, then the third region 18on the first region 14 and finally the second region 16 on the thirdregion 18. The regions are preferably deposited by liquid phase epitaxy.The regions may be sequentially deposited on the substrate by liquidphase epitaxy using the method and apparatus described in US. Pat. No.3,565,702, issued Feb. 23, 1971 to H. Nelson, entitled DepositingSuccessive Epitaxial Semiconductor Layers From The Liquid Phase."

Referring to FIG. 3, there is shown an apparatus, generally designatedas 30, which is suitable for making the semiconductor injection laser10. The apparatus 30 comprises a refractory furnace boat 32 of an inertmaterial, such as graphite. The upper surface of the boat 32 has threespaced wells 34, 36 and 38 therein. A passage 40 extends longitudinallythrough the boat 32 and crosses the bottoms of the wells 34, 36 and 38.A slide 42 of a refractory material such as graphite, movably extendsthrough the passage 40 so that the upper surface of the slide iscoplanar with the plane of the bottom of each of the wells 34, 36 and38. A recess 44 is provided in the upper surface of the slide 42adjacent one end of the slide. The recess 44 is of a size to receive thesubstrate 12 on which the regions 14, 18 and 16 are to be deposited butis slightly deeper than the thickness of the substrate.

To make the semiconductor injection laser 10, the substrate 12, whichhas a chemically polished surface, is placed in the recess 44 with thepolished surface facing upwardly. Separate charges are placed in each ofthe wells 34, 36 and 38. Each of the charges is a mixture comprising thesemiconductor material of the particular region to be deposited, a metalsolvent for the semiconductor material and a suitable dopant. Theingredients of the charges are in granulated solid form at roomtemperature. For example, if the first region 14 is to be'N type galliumaluminum arsenide, the charge in the first well 34 may comprise amixture of 5 grams gallium as the solvent, 900 milligrams galliumarsenide, milligrams aluminum and 2 milligrams tellurium as the N typedopant. If the third region 18 is to be compensated P type galliumarsenide the charge in the second well 36 may comprise a mixture of 5grams gallium, 750 milligramsgallium arsenide, 20 milligrams silicon and2 milligrams zinc. If the second region 16 is to be P type galliumaluminum arsenide, the charge in the third well 38 may comprise amixture of 5 grams gallium, 900 milligrams gallium arsenide, 10milligrams aluminum and 35 milligrams zinc as the P type dopant.

The loaded furnace boat 32 is then placed in a furnace tube (not shown)and a flow of high purity hydrogen is provided through the furnace tubeand over the furnace boat 32. The heating means of the furnace tube isturned on to heat the furnace boat 32 and its contents to a temperatureat which the charges in the wells 34, 36 and 38 become molten andsuitable for epitaxial regrowth, i.e.,approximately 900C. When thegallium becomes molten, the other ingredients of each of the charges.dissolve in the molten gallium. Thus, the charge in the first well 34becomes a first solution 46, which in this example is gallium arsenide,aluminum and the N type dopant, tellurium, dissolved in the gallium. Thecharge in the second well 36 becomes a second solution 48, which in thisexample is gallium arsenide, the amphoteric type dopant silicon and theP type dopant, zinc, dissolved in the gallium. The charge in the thirdwell 38 becomes the third solution 50, which in this example is galliumarsenide, aluminum and the P type dopant, zinc, dissolved in gallium.This temperature is maintained for a period to insure a completelyhomogeneous solution in each of the wells.

The slide 42 is then moved in the direction shown by I the arrow 52 inFIG. 3 so as to carry the substrate 12 into the first well 34 and bringthe surface of the substrate into contact with the first solution 46.The temperature of the furnace tube is then reduced so as to permit thefirst solution 46 to cool at a controlled rate. As the first solution 46cools, some of the gallium arsenide in the first solution precipitatesand deposits on the surface of the substrate 12 as the first epitaxialregion 14. Some of the aluminum in the first solution 14 becomesincorporated in the first epitaxial region 14, replacing some of thegallium ions of the gallium arsenide so that the first region is galliumaluminum arsenide. Also, some of the tellurium present in the firstsolution 46 becomes incorporated in the crystal lattice of the firstepitaxial region so that the first epitaxial region is N type galliumaluminum arsenide. in this example, the first solution 46 is cooled fromabout 900C to about 885C in a period of 2% minutes so as to provide afirst region 14 of N type gallium aluminum arsenide, Ga Al As, which isabout 3 microns in thickness.

The slide 42 is then again moved in the direction of the arrow 52 tocarry the substrate 12 from the first well 34 into the second well 36where the surface of the first region 14 is brought into contact withthe second solution 48. Some of the gallium arsenide in the secondsolution 48 precipitates and deposits on the surface of the first region14 to form the third epitaxial region 18. In this example, some of thesilicon and the zinc in the second solution 48 becomes incorporated inthe crystal lattice of the third epitaxial region 18 so that the thirdregion 18 is of compensated P type gallium arsenide. Since the thirdregion 18 is very thin, between 0.2 and 0.3 microns in thickness, thesubstrate is maintained in the second well 36 for only a short period oftime. In fact, in this example, the substrate is merely moved throughthe second well 36 without stopping with the second solution 48 being ata temperature of about 885C to achieve the third region 18 of thedesired thickness.

The slide 42 is then again moved in the direction of the arrow 52 tocarry the substrate 12 into the third well 38 where the surface of thethird region 18 is brought into contact with the third solution 50. Thereduction of the temperature of the furnace tube is continued so as tocool the third solution 50 at a controlled rate. This results in some ofthe gallium arsenide in the third solution 50 precipating and depositionon the third region 18 to form the second epitaxial region 16. As withthe first solution 46, some of the aluminum in the third solution 50becomes incorporated in the second region 16 replacing some of thegallium ions of gallium arsenide so that the second region 16 is galliumaluminum arsenide. Also, some of the zinc in the third solution 44becomes incorporated in the crystal lattice of the second region 16 sothat the second region 16 is P type gallium aluminum arsenide Ga Al As.In this example, the third solution 44 is cooled from about 885C toabout 875C in about 30 seconds so as to provide a second region 16 whichis about 1.5 mils in thickness. The slide 42 is then again moved in thedirection of the arrow 46 to carry the substrate out of the third well38 and to permit the substrate with the regions deposited thereon to beremoved from the slide.

We claim:

1. A semiconductor injection laser comprising a body of singlecrystalline semiconductor material having a first region of oneconductivity type, a second region of a conductivity type opposite tothat of the first region and a third region of either conductivity typebetween said first and second regions, the junctions between the thirdregion and each of the first and second regions being heterojunctionswhich are substantially parallel and extending to an edge of said body,the third region being of a thickness of between 0.2 and 0.3 microns andhaving an index of refraction at the lasing wavelength which is slightlyhigher but no greater than 0.05 higher than the index of refraction ofeach of the first and second regions.

2. A semiconductor laser in accordance with claim 1 in which the thirdregion contains both P type and N type dopants so as to be a compensatedregion but including more of one of the type dopants than of the othertype dopant depending on the conductivity type desired for the thirdregion.

3. The semiconductor injection laser in accordance with claim 2 in whichthe total concentration of the dopants in the third region is about "cm'and the difference of the dopant concentration between the two typedopants is about lo cm 4. The semiconductor injection laser inaccordance with claim 3 in which the dopant concentration in each of thefirst and second regions is about lo cm' 5. The semiconductor injectionlaser in accordance with claim 1 in which each of the first and secondregions are of a semiconductor material having a band gap energyslightly higher than the band gap energy of the semiconductor materialof the third region so as to provide the difference in the index ofrefraction between the third regions and each of the first and secondregions.

6. The semiconductor injection laser in accordance with claim 5 in whichthe band gap energy of the semiconductor materials of each of the firstand second regions is no greater than about 0.2eV higher than the bandgap energy of the semiconductor material of the third region.

7. The semiconductor injection laser in accordance with claim 6 in whicheach of the first and second regions are of single crystalline galliumaluminum arsenide and the third region is of single crystalline galliumarsenide.

8. The semiconductor injection laser in accordance with claim 6 in whicheach of the first and second regions are of single crystalline galliumaluminum arsenide and the third region is of single crystalline galliumaluminum arsenide having a smaller content of aluminum than thesemiconductor materials of each of the first and second regions.

1. A semiconductor injection laser comprising a body of singlecrystalline semiconductor material having a first region of oneconductivity type, a second region of a conductivity type opposite tothat of the first region and a third region of either conductivity typebetween said first and second regions, the junctions between the thirdregion and each of the first and second regions being heterojunctionswhich are substantially parallel and extending to an edge of said body,the third region being of a thickness of between 0.2 and 0.3 microns andhaving an index of refraction at the lasing wavelength which is slightlyhigher but no greater than 0.05 higher than the index of refraction ofeach of the first and second regions.
 2. A semiconductor laser inaccordance with claim 1 in which the third region contains both P typeand N type dopants so as to be a compensated region but including moreof one of the type dopants than of the other type dopant depending onthe conductivity type desired for the third region.
 3. The semiconductorinjection laser in accordance with claim 2 in which the totalconcentration of the dopants in the third region is about 1019cm 3 andthe difference of the dopant concentration between the two type dopantsis about 1017cm
 3. 4. The semiconductor injection laser in accordancewith claim 3 in which the dopant concentration in each of the first andsecond regions is about 1018cm
 3. 5. The semiconductor injection laserin accordance with claim 1 in which each of the first and second regionsare of a semiconductor material having a band gap energy slightly higherthan the band gap energy of the semiconductor material of the thirdregion so as to provide the difference in the index of refractionbetween the third regions and each of the first and second regions. 6.The semiconductor injection laser in accordance with claim 5 in whichthe band gap energy of the semiconductor materials of each of the firstand second regions is no greater than about 0.2eV higher than the bandgap energy of the semiconductor material of the third region.
 7. Thesemiconductor injection laser in accordance with claim 6 in which eachof the first and second regions are of single crystalline galliumaluminum arsenide and the third region is of single crystalline galliumarsenide.
 8. The semiconductor injection laser in accordance with claim6 in which each of the first and second regions are of singlecrystalline gallium aluminum arsenide and the third region is of singlecrystalline gallium aluminum arsenide having a smaller content ofaluminum than the semiconductor materials of each of the first andsecond regions.